Looking at the crud on the Jetta photos in this thread you need to change the oil often.
These one line replies of "extend the oci w/synthetic." and such are of NO HELP.
(and if you *read* the thread, they ARE already using it. Quote "We put synthetic in this car, in accordance with Volkswagen of USA")
Syn is not going to help the problem. EXTENDING the OCI is just a plain DUMB statement.
I can tell you the last thing Pablo or Gary would recommend after seeing those photos is a Amsoil 25k OCI. Or even a 15k one. Any they SELL the stuff.
We are here to HELP each other. Not sell stuff or push bad advise.
When you have that much moisture condensation in a crankcase that is a problem.
Two things will help.
1. Try to operate the car for a longer trip at least once a week of 20 miles or more (highway would be best).
2. CHANGE the oil. NO more than 3 months no matter HOW many miles. Use ANY SM oil rated per the manual for your engine.
Sadly, even with often oil changes this engine is not going to be a long life 300k mile motor. All the water is going to cause more wear.
But lets STOP with the one line "use synthetic oil" replies.
They DO NOT help.
Personally, I'd change the Jetta with a conventional and change it every 3 months. Change the filter same time. TRY to get it on the freeway once a week even though gas is high.
Also a used oil analysis to check if there is not a coolant leak would not be a bad idea.
Check the PCV valve too!
Take care, bill
PS: The brand of gas someone uses is not going to matter. Use fresh gas is all that matters.